Hemocyanins are oxygen transport proteins found in a variety of arthropods and mollusks. Hemocyanin from the common horseshoe crab has a molecular weight of about 3.3 million Da and is composed of 48 subunits each of 73 kDa. These subunits are eight different but related polypetides. Each subunit can reversibly bind one molecule of oxygen at an active site of two copper ions ligated by six histadine residues. The three dimensional structure of subunit II is know and available for molecular replacement phasing of the three new subunit crystal types collected at CHESS. In the 48 subunit whole hemocyanin molecule, each subunit type occupies only specific positions in the large macromolecular assembly. The subunit types differ significantly in their primary structures, ligand affinites, allosteric effectors and association properties, so comparing subunit structures will provide information about their individual functions as well as being a necessary prerequisite to understand the whole hemocyanin molecule in atomic detail.